The Chlamydia are obligate intracellular parasites of eukaryotic cells which are responsible for endemic sexually transmitted infections, trachoma, infectious pneumonitis, and various other disease syndromes. They occupy an exclusive eubacterial phylogenic branch, having no close relationship to any other known organisms—they are classified in their own order (Chlamydiales) which contains a single family (Chlamydiaceae) which in turn contains a single genus (Chlamydia). Four chlamydial species are currently known—C. trachomatis, C. pneumoniae, C. pecorum and C. psittaci [eg. see reference 1]. A genome sequence of C. trachomatis (serovar D) has recently been published [2].
The human serovariants (“serovars”) of C. trachomatis are divided into two biovariants (“biovars”). Serovars A-K elicit epithelial infections primarily in the ocular tissue (A-C) or urogenital tract (D-K). Serovars LI, L2 and L3 are the agents of invasive lymphogranuloma venereum (LGV).
Although chlamydial infection itself causes disease, it is thought that, in some patients, the severity of symptoms is due, in fact, to an aberrant host immune response. Failure to clear the infection results in persistent immune stimulation and, rather than helping the host, this results in chronic infection with severe consequences, including sterility and blindness [3].
In addition, the protection conferred by natural chlamydial infection, is usually incomplete, transient, and strain-specific.
Due to the serious nature of the disease there is a desire to provide suitable vaccines. These may be useful (a) for immunisation against chlamydial infection or against chlamydia-induced disease (prophylactic vaccination) or (b) for the eradication of an established chronic chlamydial infection (therapeutic vaccination). Being an intracellular parasite, however, the bacterium can generally evade antibody-mediated immune responses.
Various antigenic proteins have been described for C. trachomatis, and the cell surface in particular has been the target of detailed research [eg. 1,4]. These include, for instance, pgp3 [5, 6, 7], MOMP [8], Hsp60 (GroEL) [9] and Hsp70 (DnaK-like) [10]. Not all of these have proved to be effective vaccines, however, and it is an object of the invention to identify chlamydial antigens which elicit an immune response during natural infection, in order to provide antigens and immunogens suitable for use in vaccine development.